The theme of mystery could be very strong. The Reaper's origin, its purpose in the wizarding world, and what it wants from Harry and Hermione are all elements that could create a sense of mystery. It could also touch on the theme of power, as they might discover that the Reaper has some sort of power that they need to either overcome or use to their advantage.
Another theme might be redemption. The Reaper could be a character in need of redemption, or perhaps Harry and Hermione find themselves in a situation where they need to redeem someone related to the Reaper. It could also be about friendship, as they rely on each other to face this new and terrifying entity.
In this fanfiction, it's possible that the Grim Reaper appears to Harry at a crucial moment, like when he's about to make a big decision. The Grim Reaper could show him different possible futures depending on his choices, much like a guide through the complex web of time and consequences. It could also be that the Grim Reaper has a connection to Voldemort, and Harry has to deal with this new and unexpected alliance or conflict.
One possible plot could be that the Reaper has come to collect a soul in Hogwarts, and Harry and Hermione, being the brave ones, try to stop it. They might discover that the Reaper is not all - evil as they thought and there could be some dark magic involved that they need to deal with together.
A very common theme is the power of teamwork. Harry and Hermione are both intelligent and resourceful, and in fanfiction, they often combine their skills to overcome great obstacles. Another theme is love, not necessarily romantic love, but love in the sense of caring deeply for each other. It could be platonic love that gets tested in difficult situations, like when they have to make sacrifices for each other in the face of danger.
There might be a new character introduced in the fanfiction who is using the appearance of the Grim Reaper to cause chaos. This character could be a dark wizard who has found a way to mimic the Grim Reaper's powers and uses it to spread fear among the wizarding community. Harry would then have to figure out who this impostor is and stop their evil deeds.
There are so many things that could occur. For instance, Harry's parents could be curious about Hermione's relationship with their son. They might ask her about her family and her magical abilities. Hermione, on the other hand, could be eager to learn from them about Harry's early life. It could be a very emotional encounter, with lots of reminiscing about Harry as a baby and how much they missed seeing him grow up. And they might also share some secrets about the Potter family that could help Hermione and Harry in their later battles.
In this fanfiction, Harry might be nervous at first. He could be worried about making a good impression on Hermione's family. Maybe he brings a special gift, like a magical book he found. And he might be surprised by how warm and welcoming they are.
One possible plot could be that they meet in a muggle library. Hermione is there researching magic in the guise of general knowledge books, and Harry stumbles upon her while looking for a quiet place to escape from the Dursleys. They start chatting about books, and Hermione is intrigued by Harry's strange yet endearing manner. They then realize they both have a connection to the magical world.
Well, Hermione's parents could be key characters. They might be the ones who first notice Harry's odd situation when they meet him. For example, they could see that Harry seems a bit underfed and neglected, which could lead to them trying to help him in some small way. This would also give Hermione more of a connection to Harry before Hogwarts.
The consequences could be dire. Reapers in the Black Butler universe are not to be trifled with. They are tasked with collecting souls and maintaining the balance of life and death. Summoning one might disrupt that balance. For example, the reaper may take more souls than intended in the area as a result of being summoned, causing chaos among the living. It could also lead to the summoner, Harry in this case, getting into trouble with other supernatural entities who don't want the natural order meddled with.
In the fanfic, they might start by discussing the magical creatures in the zoo. Maybe they'll share their thoughts on how muggles perceive these animals without knowing their true magical nature.